This section is intended to provide a background or context to the invention that is, inter alia, recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Illicit radioactive material can potentially appear in a wide range of public venues. Large national public events are routinely monitored for such radioactive sources. One element in a plan for managing such threats involves searching for sources in venues at risk and tracking the sources in real time when they are detected. A radiation source that can be tracked unobtrusively potentially yields useful information such as the purveyor's identity and, for future threat reduction, the means by which the source was deployed. In addition, real-time incident-specific information is of use to on-site security personnel.
Two examples of environments that can benefit from monitoring include the confined-venue (e.g. a stadium or a convention center) and the wide-area venue. In the confined-venue problem pedestrian traffic enters through a limited number of entrances or what are effectively choke points. Screening for radiation sources can generally be achieved with a few radiation detectors located proximate the choke points. For example, because the distances are relatively short, allowing coverage by a few well placed fixed detectors, thereby eliminating the manpower requirements of mobile detectors, integrated fixed isotropic detectors may be used. On the other hand, in the wide-area problem access to the venue is less controlled both in time and space. For example, a ship boarding party inspecting a vessel and its cargo whose disposition is not well characterized is unable to take advantage of choke points to locate a radiation source. Further examples of wide-area problems include large open-air events, such as an inauguration, having many pedestrian approaches, some of which are large spaces with no choke point. The differences between the confined-venue problem and the wide-area problem have implications for the types of detectors most suited for monitoring of radiation sources within those environments.
A directional-detector array is generally known within the nuclear threat-reduction community. Such a radiation detector array may process measured count data by taking simple ratios of counts without treatment of various uncertainties and ambient conditions within the environment. Significantly, existing radiation detection systems are not capable of estimating an (x,y) position for a radiation source. Although existing directional detectors can estimate a direction, they cannot provide a probability of locating a source as a function of direction. Current directional-detector technology is further limited as such systems generally operate as a standalone unit without the capability for sharing data among several units to improve the accuracy of the source position estimate. Existing directional detection methods also do not provide an estimate for direction uncertainty. Other directional detectors, including the Compton-scattering camera, the coded-aperture imager, and positron emission backscatter imaging, suffer from similar deficiencies.
Accordingly, there is a need to improve current radiation detection, search and tracking capabilities while reducing the cost of the detection equipment and deployment of the equipment. To date, emphasis has largely been placed on developing and engineering advanced detector materials to provide greater measurement sensitivity. More recently, the use of various algorithms to tease more information out of detector signals has received interest. One such area involves combining the signals from multiple detectors to improve sensitivity and the position estimate. However, many existing systems provide only a simple alarming capability on a detector-by-detector basis and leave it to the operator to infer source position. Additionally, a fundamental problem that generally underlies radiation source tracking is an inability to achieve acceptable position estimate accuracy in an environment where the signal-to-noise ratio is low, which may preclude obtaining an estimate of the source position.